Problem # 3-35 from Chapter 3 in book "Introduction to Optics" by
Pedrotti, second edition.

An airplane is used in aerial surveying to make a map of ground
detail. If the scale of the map is to be 1:50,000 and the camera used
has a focal length of 6 inch, determine the proper altitude for the
photograph.

Hello al3in, this is a very simple problem if you know the definition
of focal length. I didn't, but was curious:

http://interactive2.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=343

Focal length is the distance measured from camera lens to film.
Knowledge of the focal length used, along with the altitude of the
photographic aircraft, makes it possible to determine the scale of the
aerial photograph (assuming land uniformity). For example, with a
camera focal length of six inches, a flight altitude of 6,500 feet
above mean sea level, and an average ground elevation of 1,500 feet,
the representative fraction would be computed as follows:

0.5 ft. divided by (6,500 ft. - 1,500 ft.) = 1:10,000 scale

Or, in your case:

0.5 ft. (6 inches) / altitude = scale

altitude = 0.5 / scale

altitude = 0.5 / (1/50,000) = 0.5 x 50,000

altitude = 25,000 ft.

Now we both understand focal length, Redhoss

Perfect answer, Thanks alot.